A variety of fibers and fabrics have been made from thermoplastics, such as polypropylene, highly branched low density polyethylene (LDPE) made typically in a high pressure polymerization process, linear heterogeneously branched polyethylene (for example, linear low density polyethylene made using Ziegler catalysis), linear and substantially linear homogeneously branched polyethylene, blends of polypropylene and linear heterogeneously branched polyethylene, blends of linear heterogeneously branched polyethylene, and ethylene/vinyl alcohol copolymers.
Fiber is typically classified according to its denier (gms/9000 m). Monofilament fiber is generally defined as having an individual fiber denier greater than about 14. Fine denier fiber generally refers to a fiber having a denier less than about 10 denier per filament. Microdenier fiber is generally defined as fiber less than 1 denier or less than 10 microns.
The fiber can also be classified by the process by which it is made, such as monofilament, continuous wound fine filament, staple or short cut fiber, spun bond, and melt blown fiber.
Many polyolefin materials are known to be useful in the formation of fiber. Linear heterogeneously branched polyethylene has been made into monofilament, as described in U.S. Pat. No. 4,076,698 (Anderson et al.). Linear heterogeneously branched polyethylene has also been successfully made into fine denier fiber, as disclosed in U.S. Pat. No. 4,644,045 (Fowells), U.S. Pat. No. 4,830,907 (Sawyer et al.), U.S. Pat. No. 4,909,975 (Sawyer et al.) and in U.S. Pat. No. 4,578,414 (Sawyer et al.). Blends of such heterogeneously branched polyethylene have also been successfully made into fine denier fiber and fabrics, as disclosed in U.S. Pat. No. 4,842,922 (Krupp et al.), U.S. Pat. No. 4,990,204 (Krupp et al.) and U.S. Pat. No. 5,112,686 (Krupp et al.). U.S. Pat. No. 5,068,141 (Kubo et al.) also discloses making nonwoven fabrics from continuous heat bonded filaments of certain heterogeneously branched LLDPE having specified heats of fusion.
However, fibers made from all of these types of saturated olefinic polymers are not naturally “elastic” (as that term is defined below) thus limiting their use in elastic applications. One attempt to alleviate this problem by incorporating additives into the polymer prior to melt spinning is disclosed in U.S. Pat. No. 4,663,220 (Wisneski et al.). Wisneski et al. disclose fibrous elastomeric webs comprising at least about 10 percent of a styrenic block copolymer and a polyolefin. The resultant webs are said to have elastomeric properties.
U.S. Pat. No. 4,425,393 (Benedyk) discloses monofilament fiber made from polymeric material having an elastic modulus from 2,000 to 10,000 psi. The polymeric material includes plasticized polyvinyl chloride (PVC), low density polyethylene (LDPE), thermoplastic rubber, ethylene-ethyl acrylate, ethylene-butylene copolymer, polybutylene and copolymers thereof, ethylene-propylene copolymers, chlorinated polypropylene, chlorinated polybutylene or mixtures of those.
Elastic fiber and web prepared from a blend of at least one elastomer (that is, copolymers of an isoolefin and a conjugated polyolefin (for example, copolymers of isobutylene and isoprene)) and at least one thermoplastic is disclosed in U.S. Pat. No. 4,874,447 (Hazelton et al.).
U.S. Pat. No. 4,657,802 (Morman), discloses composite nonwoven elastic webs and a process for their manufacture. The elastic materials useful for forming the fibrous nonwoven elastic web include polyester elastomeric materials, polyurethane elastomeric materials, and polyamide elastomeric materials.
U.S. Pat. No. 4,833,012 (Makimura et al.), discloses nonwoven entanglement fabrics made from a three dimensional entanglement of elastic fibers, nonshrinkable nonelastic fibers, and shrinkable elastic fibers. The elastic fibers are made from polymer diols, polyurethanes, polyester elastomers, polyamide elastomers and synthetic rubbers.
Composite elastomeric polyether block amide nonwoven webs are disclosed in U.S. Pat. No. 4,820,572 (Killian et al.). The webs are made using a melt blown process and the elastic fibers are made from a polyether block amide copolymer.
Another elastomeric fibrous web is disclosed in U.S. Pat. No. 4,803,117 (Daponte). Daponte discloses that the webs are made from elastomeric fibers or microfibers made from copolymers of ethylene and at least one vinyl monomer selected from the group including vinyl ester monomers, unsaturated aliphatic monocarboxylic acids and alkyl esters of these monocarboxylic acids. The amount of the vinyl monomer is said to be “sufficient” to impart elasticity to the melt-blown fibers. Blends of the ethylene/vinyl copolymers with other polymers (for example, polypropylene or linear low density polyethylene) are also said to form the fibrous webs.
While previous efforts to make elastic fibers and fabrics from olefinic polymers have focused on polymer additives, these solutions have potential detriments, including the increased cost of the additives, and substandard spinning performance.
More recently, elastic fibers made from polyolefin materials and particularly crosslinked polyolefin materials, such as those disclosed in U.S. Pat. Nos. 5,824,717; 6;048,935; 6,140,442; 6,194,532; 6,437,014, 6,500,540, and 6,500,540 have received much attention, particularly in the field of textiles and apparel. The crosslinked, olefin elastic fibers include ethylene polymers, propylene polymers and fully hydrogenated styrene block copolymers (also known as catalytically modified polymers). The ethylene polymers are preferred for many applications and include the homogeneously branched and the substantially linear homogeneously branched ethylene polymers as well as ethylene-styrene interpolymers. These crosslinked, olefin elastic fibers have been lauded for their chemical and heat resistance, their durability and their comfort stretch, and they are accordingly growing in popularity in both weaving and knitting applications.
The superior properties of these crosslinked olefin elastic fibers have led to their commercial success. However, it has been reported that such fibers still experience a rate of breaking which is higher than desired during downstream processing of fibers. Fiber breaks occur during bobbin formation, spool unwinding and winding, drafting (during yarn making or covering), cone dyeing, and at friction points during knitting operations. While the rate of fiber breakage is commercially acceptable, it could still be improved. Accordingly, it is a goal of the present invention to provide a more robust crosslinked polyolefin elastomeric fiber, to further reduce the rate of occurrence of downstream fiber breaks. This goal must be balanced against other interests, however. In particular the goal must not come at the expense of acceptable fiber processing characteristics. Properties such as good spinnability, good elongation to break, retractive force, crosslinkability, tackiness and temperature resistance, must remain acceptable.
It has been discovered that using a composition comprising a polyolefin blend having a melt index (I2) less than 2.5 g/10 min with a density in the range of 0.86 to 0.89 g/cm3 improves the tenacity of the fiber while avoiding tackiness and preserving the elastic behavior. The compositions for use in the present inventions comprise at least two components. The components can be classified according to the following characteristics. Characteristic (a) is that the polyolefin material has a density in the range of 0.855 to 0.880 g/cm3. Characteristic (b) is that the polyolefin material has a residual crystallinity at 80° C. greater than or equal to 9 percent. It is believed that materials meeting characteristic (a) impart elasticity and crosslinkability to the fiber whereas material meeting characteristic (b) impart heat stability to the fiber. For the fibers of the present invention, blends of two or more polyolefin components are used where at least one of the components meets either (a) or (b) but not both. The second component is selected such that it will meet whichever characteristic ((a) or (b)) the first component does not meet. It is within the scope of the invention that the second component can meet only one of these characteristics or both simultaneously.
The fibers made from such materials exhibit improved retractive power, which leads to better properties of the fiber at ambient temperature and better dimensional stability at higher temperatures.
Despite the belief among those skilled in the art that higher molecular weight materials results in higher spinline stress and therefore more breaks, it has surprisingly been observed that the compositions of the present invention exhibit excellent spinnability, both in terms of the processability in an extruder and in terms of the drawability of the melt after exiting the extruder.